1. Field of the Invention
The present invention relates, in general, to a method for making a field oxide of a semiconductor device and, more particularly, to a local oxidation of silicon (hereinafter referred to as "LOCOS") process by which the wafer warpage of a large wafer is minimized.
2. Description of the Prior Art
A LOCOS process is one of the typical methods for making a field oxide. According to the process, a pad oxide and a silicon nitride are sequentially laminated over a substrate, a predetermined region of the nitiride is removed and the substrate is oxidized at the predetermined region, to form a field oxide.
Since such LOCOS process is simple and is found to cause much fewer defects compared with other processes, it is the most widely used for mass production. However, the LOCOS process cannot be directly applied for highly integrated devices because it has the significant problem of a long bird's beak. To avoid this bird's beak problem, a modified LOCOS process has been recently developed, in which the substrate is etched at the nitride-removed region to a predetermined thickness to form a trench or groove at which a subsequent oxidation process is proceeded.
Today, wafers with large diameters are employed in order to increase productivity. However, new problems other than the bird's beak occur in the LOCOS process when using a wafer 200 mm (8 inches) or greater in size. One of the most representative examples is wafer warpage (an index indicating crookedness). Wafer warpage is not problematic at all for the wafers as small as 150 mm (6 inches), but arises for large-sized wafers.
In FIG. 1, wafer warpage which is generated while a field oxide is formed on a wafer 200 mm or greater in diameter is shown. In the course of forming a field oxide in a LOCOS process, pad oxides 2 and 2' are thermally grown on the front and the back sides of a wafer 1 and nitrides 3 and 3', resistant to oxidation, are deposited in a low pressure chemical vapor deposition (LPCVD) technique. These LPCVD nitrides are formed on the front side F and the back side B, at once. Subsequently, the nitride 3 on the front side is locally removed at predetermined field regions by an etching process. As seen, the nitride 3 on the front side F is partly removed and separated like islands whereas the nitride on the back side B remains continuous. At this time, this continuous nitride strongly exerts its tensile stress on the back side of the wafer, making the wafer bend (wafer warpage). This wafer warpage is more serious as the diameter of the wafer is larger.
To solve such wafer warpage as occurs owing to the above-mentioned mechanism, a plasma enhanced CVD (PECVD) technique was developed by which a nitride was deposited on the front side only. However, a PECVD nitride cannot be used in practical element-isolation processes because it is far inferior to an LPCVD nitride in various film properties including oxidation resistance.
With reference to FIG. 2, there are data of an experiment in which wafer warpage is measured according to the thicknesses of a nitride used as an element-isolating mask in a wafer 200 mm in diameter. From the data, wafer warpage is found to be largely modulated by the diameter of a wafer as well as the thickness of the nitride deposited. Just after the deposition of nitride (B), there is a valence between the stresses on the front and the back sides, so that slight warpage forms and is little changed with the thickness of nitride. However, after the nitride at the field regions on the front side is removed (A), warpage is 2-3 folds increased and is aggravated by the thickness of nitride. In particular, when the deposited nitride is thicker than 1,500 Angstrom, warpage is abruptly raised.
The following are the problems of large wafer warpage.
First, if a wafer is bent, a significant misalignment in the photo processes which proceed after the element isolation process occurs and the uniformity in critical dimension (CD) is lowered among the dies within the wafer.
In addition, when a gate oxide is formed after the element isolation process, a bent wafer gives poor properties to the gate oxide.
FIG. 3, shows a bar graph. It is obtained from an experiment in which a property of the gate oxide formed on a 200 mm wafer following the element-isolation process is investigated while the nitride to be used in the element-isolation process is changed in thickness from 1000 to 2500 Angstrom. The Y axis shows the percentage ratio of the die number of good gate oxide to the total die number. The data demonstrated that the die number of good gate oxide is reduced as the nitride is thicker. This is attributed to the fact that the wafer warpage increases with the thickness of the nitride.
As apparent from the data, the thickness of the nitride should be not more than 1500 Angstrom in order to improve the reliability of the gate oxide. In this case, however, another problem occurs in practice. One of the most important requirements for the element isolation process is to shorten the bird's beak. The length of the bird's beak of field oxide formed in LOCOS process is sensitively affected by the thickness of the nitride. That is, the reduction of the thickness of the nitride into a critical value is disadvantageous in the aspect of the bird's beak.